Systems and methods for improved medical imaging

ABSTRACT

A radiation detector assembly is provided that includes a semiconductor detector, plural pixelated anodes, and at least one processor. The semiconductor detector has a surface. The plural pixelated anodes are configured to generate a primary signal responsive to reception of a photon by the pixelated anode and to generate at least one secondary signal responsive to an induced charge caused by reception of a photon by at least one surrounding anode. The at least one processor is configured to: acquire a primary signal from one of the anodes responsive to reception of a photon; acquire at least one secondary signal from at least one neighboring pixel of the one of the anodes, the at least one secondary signal defining a negative value; and determine an energy correction factor for the reception of the photon using the negative value of the at least one secondary signal.

RELATED APPLICATONS

The present application is a continuation-in-part of, and claimspriority to, U.S. patent application Ser. No. 16/102,228, entitled“Systems and Methods for Determination of Depth of Interaction,” filedAug. 13, 2018, the subject matter of which is hereby incorporated in itsentirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to apparatus andmethods for diagnostic medical imaging, such as Nuclear Medicine (NM)imaging.

In NM imaging, for example, systems with multiple detectors or detectorheads may be used to image a subject, such as to scan a region ofinterest. For example, the detectors may be positioned adjacent thesubject to acquire NM data, which is used to generate athree-dimensional (3D) image of the subject.

Imaging detectors may be used to detect reception of photons from anobject (e.g., human patient that has been administered a radiotracer) bythe imaging detector. The depth of interaction (DOI) or location alongthe thickness of a detector at which photons are detected may affect thestrength of the signals generated by the detector responsive to thephotons and be used to determine the number and location of detectedevents. Accordingly, the DOI may be used to correct detector signals toimprove detector energy resolution and sensitivity. However,conventional approaches to determining DOI utilize signals from acathode, requiring additional hardware and assembly complexity toutilize hardware to collect and process cathode signals. Also, cathodestend to be relatively large and produce relatively noisy signals,reducing the accuracy and effectiveness of using signals from cathodes.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a radiation detector assembly is provided thatincludes a semiconductor detector, plural pixelated anodes, and at leastone processor. The semiconductor detector has a surface. The pluralpixelated anodes are disposed on the surface, with each pixelated anodeconfigured to generate a primary signal responsive to reception of aphoton by the pixelated anode and to generate at least one secondarysignal responsive to an induced charge caused by reception of a photonby at least one surrounding anode. The at least one processor isoperably coupled to the pixelated anodes, and configured to: acquire aprimary signal from one of the anodes responsive to reception of aphoton by the one of the anodes; acquire at least one secondary signalfrom at least one neighboring pixel of the one of the anodes responsiveto an induced charge caused by the reception of the photon by the one ofthe anodes, the at least one secondary signal defining a negative value;and determine an energy correction factor for the reception of thephoton by the one of the anodes using the negative value of the at leastone secondary signal.

In another embodiment, a method of imaging using a semiconductordetector is provided. The semiconductor detector has a surface withplural pixelated anodes disposed thereon. Each pixelated anode isconfigured to generate a primary signal responsive to reception of aphoton by the pixelated anode and to generate at least one secondarysignal responsive to an induced charge caused by reception of a photonby at least one surrounding anode. The method includes acquiring aprimary signal from one of the anodes responsive to reception of aphoton by the one of the anodes. The method also includes acquiring atleast one secondary signal from at least one neighboring pixel of theone of the anodes responsive to an induced charge caused by thereception of the photon by the one of the anodes. The at least onesecondary signal defines a negative value. Further, the method includesdetermining an energy correction factor for the reception of the photonby the one of the anodes using the negative value of the at least onesecondary signal.

In another embodiment, method is provided for providing a radiationdetector assembly. The method includes providing a semiconductordetector having a surface with plural pixelated anodes disposed thereon.Each pixelated anode is configured to generate a primary signalresponsive to reception of a photon by the pixelated anode and togenerate at least one secondary signal responsive to an induced chargecaused by reception of a photon by at least one adjacent anode. Themethod also includes operably coupling the pixelated anodes to at leastone processor. Further, the method includes providing a calibratedradiation supply to the semiconductor detector. The pixelated anodesgenerate primary signals and secondary signals responsive to thecalibrated radiation supply. Also, the method includes acquiring, withthe at least one processor, the primary signals and the secondarysignals from the pixelated anodes. The method also includes determiningcorresponding negative values of total induced charges for the secondarysignals corresponding to different depths of absorption, and determiningcalibration information based on the negative values of the totalinduced corresponding to the different depths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a representation of weighting potentials of a detectorhaving a pixel biased by a voltage potential.

FIG. 2 depicts four events within the detector of FIG. 1.

FIG. 3 depicts corresponding induced charges for the four events of FIG.2.

FIG. 4 depicts five groups of events under a primary or collecting pixellocated at five different DOI's.

FIG. 5 depicts the resulting non-collected or secondary signals for theevents located at Z₀ of FIG. 4.

FIG. 6 depicts the resulting non-collected or secondary signals for theevents located at Z₁ of FIG. 4.

FIG. 7 depicts the resulting non-collected or secondary signals for theevents located at Z₂ of FIG. 4.

FIG. 8 depicts the resulting non-collected or secondary signals for theevents located at Z₃ of FIG. 4.

FIG. 9 depicts the resulting non-collected or secondary signals for theevents located at Z₄ of FIG. 4.

FIG. 10 depicts a calibration system in accordance with variousembodiments.

FIG. 11 provides a schematic view of a radiation detector assembly inaccordance with various embodiments.

FIG. 12 provides a flowchart of a method in accordance with variousembodiments.

FIG. 13 provides a flowchart of a method in accordance with variousembodiments.

FIG. 14 provides a schematic view of an imaging system in accordancewith various embodiments.

FIG. 15 provides a schematic view of an imaging system in accordancewith various embodiments.

FIG. 16A schematically illustrates a radiation detector assembly inaccordance with various embodiments.

FIG. 16B provides a graph showing signals produced by a primary pixel ofthe radiation detector assembly of FIG. 16A.

FIG. 16C provides a graph showing signals produced by a secondary pixelof the radiation detector assembly of FIG. 16A.

FIG. 17 provides a graph containing example empirical functions F(S)that may be derived from the radiation detector assembly of FIG. 16A bymeasuring and correlating the values of the primary signals E and thenegative values S of the secondary induced signals.

FIG. 18 schematically illustrates multiple spectra produced by theradiation detector assembly of FIG. 16A.

FIG. 19 is a schematic illustration illustrating application of acorrection factor K on the energies of the events to improve theenergy-resolution and the sensitivity of the radiation detector assemblyof FIG. 16A.

FIG. 20 provides a flowchart of a method in accordance with variousembodiments.

FIG. 21 provides a flowchart of a method in accordance with variousembodiments.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of certain embodiments will be betterunderstood when read in conjunction with the appended drawings. To theextent that the figures illustrate diagrams of the functional blocks ofvarious embodiments, the functional blocks are not necessarilyindicative of the division between hardware circuitry. For example, oneor more of the functional blocks (e.g., processors or memories) may beimplemented in a single piece of hardware (e.g., a general purposesignal processor or a block of random access memory, hard disk, or thelike) or multiple pieces of hardware. Similarly, the programs may bestand alone programs, may be incorporated as subroutines in an operatingsystem, may be functions in an installed software package, and the like.It should be understood that the various embodiments are not limited tothe arrangements and instrumentality shown in the drawings.

As used herein, the terms “system,” “unit,” or “module” may include ahardware and/or software system that operates to perform one or morefunctions. For example, a module, unit, or system may include a computerprocessor, controller, or other logic-based device that performsoperations based on instructions stored on a tangible and non-transitorycomputer readable storage medium, such as a computer memory.Alternatively, a module, unit, or system may include a hard-wired devicethat performs operations based on hard-wired logic of the device.Various modules or units shown in the attached figures may represent thehardware that operates based on software or hardwired instructions, thesoftware that directs hardware to perform the operations, or acombination thereof.

“Systems,” “units,” or “modules” may include or represent hardware andassociated instructions (e.g., software stored on a tangible andnon-transitory computer readable storage medium, such as a computer harddrive, ROM, RAM, or the like) that perform one or more operationsdescribed herein. The hardware may include electronic circuits thatinclude and/or are connected to one or more logic-based devices, such asmicroprocessors, processors, controllers, or the like. These devices maybe off-the-shelf devices that are appropriately programmed or instructedto perform operations described herein from the instructions describedabove. Additionally or alternatively, one or more of these devices maybe hard-wired with logic circuits to perform these operations.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of are not intended to beinterpreted as excluding the existence of additional embodiments thatalso incorporate the recited features. Moreover, unless explicitlystated to the contrary, embodiments “comprising” or “having” an elementor a plurality of elements having a particular property may includeadditional elements not having that property.

Various embodiments provide systems and methods for improving thesensitivity and/or energy resolution of image acquisition, for examplein Nuclear Medicine (NM) imaging applications. In various embodiments,measurements of non-collected (or induced) adjacent transient signalsare utilized to determine a depth of interaction (DOI) of correspondingevents in a detector. It may be noted that the same measurements of thenon-collected adjacent transient signals may also be used to determinecorresponding sub-pixel locations for the events.

Generally, various embodiments provide methods and/or systems formeasuring the negative value of induced signals and deriving ordetermining the corresponding DOI based on the negative value. Forcertain values of DOI (e.g., DOI's not located near an anode), allevents having the same DOI produce about the same negative value fornon-collected induced signals, regardless of their lateral position(with lateral position defined as x, y coordinates where the DOI ismeasured along a z axis). Accordingly, various embodiments use ameasured value of induced signals (e.g., a measured negative value ofnon-collected induced signals, which may also be used for sub-pixellocation determination) to derive or determine the DOI as well as toprovide 3D positioning of events causing the non-collected inducedsignals.

A technical effect provided by various embodiments includes increasedsensitivity and/or energy resolution of a detector system, such as a NMimaging detector system. A technical effect of various embodimentsincludes improved image quality. A technical effect of variousembodiments includes reduced processing and/or hardware complexityassociated with determining DOI via the elimination of use of signalsfrom a cathode.

Before addressing specific aspects of particular embodiments, certainaspects of detector operation are discussed. FIG. 1 depicts arepresentation 10 of weighting potentials for a detector 11 having apixelated anode 12 biased by a potential of 1 volt. Neighboringpixelated anodes 13 are not biased, or are at a ground potential of 0volts. It may be noted that the depiction of a given pixelated anode ata voltage whereas neighboring pixelated anodes are not biased inconnection with various examples herein is provided for clarity ofillustration and ease of depiction; however, in practice each pixelatedanode of a detector may be biased by a similar voltage. In the exampledepicted in FIG. 1, the cathode 14 is grounded at 0 volts. Solid curves15 show lines of the electrical field, with dashed curves 16 showinglines of equipotential. The lines of equipotential are perpendicular tothe lines of electrical field at the points where the lines cross.

The weighting potentials of FIG. 1 are depicted according to theShockley-Ramo theorem. Under this theorem, the induced current producedby the weighting potential is described by i=qE*V=qE*V*cos(a), where iis the induced current, q is the electron charge, and E*V is the scalarproduct between the electrical field E of the weighting potential andthe velocity V of the electron, and a is the angle between the vectors Eand V.

FIGS. 2 and 3 depict the occurrence of events in various locations ofthe detector 11 and resulting induced charges. FIG. 2 depicts fourevents within the detector 11 of FIG. 1, and FIG. 3 depictscorresponding induced charges.

FIG. 2 depicts four events—event 21 starting at a depth of Z₀, event 22starting at a depth of Z₁, event 23 starting a depth of Z₂, and event 24starting at a depth of Z₃. Each of the events moves along a trajectorystarting at X₁ and ending at the anode of the primary or collectingpixel 25 (the anode that collects the events). The non-collected inducedcharge on the adjacent pixel (or non-collecting pixel, in this casepixel 12, which is immediately neighboring the collecting pixel 25 inthe illustrated example) is the integral over time (or over distance) ofthe current given by the relationship discussed above(i=qE*V=qE*V*cos(a)). E is the field of the weighting potential of thenon-collecting adjacent pixel (pixel 12 in this example). Two ranges areshown over the depth D of the detector—a first range I, and a secondrange II.

In range I, the vector of the field has a component directed downward.Accordingly, the induced charge is positive over range I. Over range II,the vector of the field has a component that is directed upward.Accordingly, the induced charge is negative over range II. Event 21 atdepth Z₀ starts at the cathode 14 and accordingly the related chargetravels over the entire length of range I and range II. The other eventsstart away from the cathode and accordingly the related charges do nottravel over the entire depth of range I. Further, the event 24 at depthZ₃ starts within the boundary of range II, closer to the pixelatedanodes than is the boundary of range II. Accordingly, the charge relatedto event 24 at depth Z₃ does not travel over the entire depth of rangeII.

FIG. 3 depicts resulting signals corresponding to the events of FIG. 2.Namely, signal 32 depicts the collecting or primary signal resulting inthe collecting pixel 25. Signal 34 depicts the non-collecting signal ofpixelated anode 12 resulting from event 21 starting at Z₀, signal 35depicts the non-collecting signal of pixelated anode 12 resulting fromevent 22 starting at Z₁, signal 36 depicts the non-collecting signal ofpixelated anode 12 resulting from event 23 starting at Z₂, and signal 37depicts the non-collecting signal of pixelated anode 12 resulting fromevent 24 starting at Z₃.

As seen in FIG. 3, for the event 21 starting at Z₀ (the event occurringat the cathode 14 and traveling over the entire range of both range Iand II), the total induced charge in ranges I and II is zero, as thepositive induced charge in range I and the negative induced charge inrange II are equal and cancel each other out over the full depth (e.g.,at the anodes where Z=D, where D is the thickness of detector 11).

For the event 22 starting at Z₁ (which is away from the cathode), thetotal induced charge is negative, as the positive induced charge inrange I is less than that from event 21, as the charge from event 22does not traverse the entire depth of range I. Similarly, the totalinduced charge from event 23 is more negative than the total inducedcharge from event 22, and the total induced charge from event 24 is morenegative than the total induced charge from event 23. This may berepresented as [Q₀=0]>[Q₁<0]>[Q₂<0]>[Q₃<0], where Q₀ is the totalinduced charge for event 21 starting at Z₀, where Q₁ is the totalinduced charge for event 22 starting at Z₁, where Q₂ is the totalinduced charge for event 23 starting at Z₂, and where Q₃ is the totalinduced charge for event 24 starting at Z₃, Accordingly, as seen in FIG.3, the closer an event is to the pixelated anodes (or farther from thecathode), the more negative the signal from a non-collected anode willtend to be.

FIG. 4 depicts five groups of events under a primary pixel 42 (thecollecting pixel that generates a primary signal) located at fivedifferent DOI's: Z₀, Z₁, Z₂, Z₃, Z₄. Each group includes three eventslocated at different X coordinates (namely X₁, X₂, and X₃) for aparticular depth. The trajectories for each event moving toward theprimary anode 42 (e.g., anode 25 of FIG. 2) are schematically shown inFIG. 4. These events move in the weighted potential and electrical fieldof the adjacent non-collecting pixel 44 (e.g., anode 12 of FIGS. 1 and2) on which the non-collected charge is induced, resulting in asecondary signal generated by the adjacent, non-collecting pixel 44.

The resulting induced non-collected or secondary signals produced byeach event are depicted in FIGS. 5-9. FIG. 5 includes a graph 50 thatdepicts the resulting non-collected or secondary signals for the eventslocated at Z₀. As seen in FIG. 5, despite the fact that the group ofevents at Z₀ have different lateral locations X₁, X₂, and X₃, they allproduce the same total non-collected induced charge signal that is equalto zero. It may be noted that all of the events at Z₀ start at thecathode. Curve 51 in FIG. 5 is the primary collected signal at primaryanode 42, and is shown in FIG. 5 to help illustrate the differencesbetween the primary signal and the secondary induced signal in terms ofamplitude and shape.

FIG. 6 includes a graph 60 depicts the resulting non-collected orsecondary signals for the events located at Z₁. As seen in FIG. 6,despite the fact that the group of events at Z₁ have different laterallocations X₁, X₂, and X₃, they all produce the same (or very nearly thesame) total non-collected induced charge signal that is negative. It maybe noted that all of the events at Z₁ start a small distance from thecathode and accordingly have a negative induced charge of a relativelysmall magnitude.

FIG. 7 includes a graph 70 depicts the resulting non-collected orsecondary signals for the events located at Z₂. As seen in FIG. 7,despite the fact that the group of events at Z₂ have different laterallocations X₁, X₂, and X₃, they all produce about the same totalnon-collected induced charge signal that is negative (e.g., within range72). It may be noted that all of the events at Z₂ start a largerdistance from the cathode than for the event at Z₁ and accordingly havea relatively more negative induced charge of a relatively smallmagnitude.

FIG. 8 includes a graph 80 that depicts the resulting non-collected orsecondary signals for the events located at Z₃. As seen in FIG. 8,despite the fact that the group of events at Z₃ have different laterallocations X₁, X₂, and X₃, they all produce about the same totalnon-collected induced charge signal that is negative (e.g., within range82). It may be noted that all of the events at Z₃ start a largerdistance from the cathode than for the event at Z₂ (and Z₁) andaccordingly have a relatively more negative induced charge of arelatively small magnitude. It may be noted that the difference betweenthe upper and lower value for the range 82 and the range 72 (see FIG. 7)are small enough to be ignored in various embodiments, so that the DOIis treated as independent of the lateral location.

FIG. 9 includes a graph 90 that depicts the resulting non-collected orsecondary signals for the events located at Z₄. The negative charges forthe events originating at a depth of Z₄ are substantially different fromeach other, due to the proximity of Z₄ to the anodes. Generally, in theillustrated example, as the depth of the event moves closer to theanode, the variability in the negative induced charge based on lateralposition increases, with the variability becoming substantial only atdepths very close to the anode.

As discussed above, except for events that start very close to acollecting anode, events produce a total non-collected induced charge inone or more adjacent anodes to the collecting anode that is correlatedto the DOI of the event, substantially independent of lateral position.Accordingly, the total induced charge caused by an event on the adjacentpixelated anode (or pixelated anodes) that is zero or negative may beutilized for deriving or determining the DOI of the particular event. Itmay further be noted that due to the high absorption of the detector,very few events start close to the anodes, and accordingly such eventsmay have a negligible effect on the use of negative induced charge todetermine DOI. Various embodiments and methods disclosed hereinaccordingly determine a magnitude for a negative induced non-collectedsignal (also referred to herein as a secondary signal), and use thedetermined negative signal magnitude value to determine or derive DOI.

As discussed above, in various embodiments the DOI of an event may bederived from a correlation between the DOI of the event and the totalnon-collected induced charge on the adjacent pixel (or pixels) which iszero or negative, with the correlation between the DOI and the totalnon-collected induced charge being substantially independent of lateralposition, such that lateral position may be disregarded in deriving theDOI. However, it may be noted that, for example, different photons mayhave different energies that can produce a different value for the totalnon-collected induced charge. Accordingly, in various embodiments, adetector system may be calibrated to account for different photonenergies, for example to normalize the non-collected induced chargevalue to photon energy. Such a calibration process may be performed toprovide calibration information that is used to determine DOI. Thecalibration information may be in the form of a look-up table, as oneexample, or in the form of a formula or mathematical expression based ona curve fitting, as another example.

FIG. 10 depicts a calibration system 92 in accordance with anembodiment. The depicted calibration system 92 is utilized to calibratea detector 93 having a sidewall 94 that extends between an anode surface95 and a cathode surface 96. The calibration system 92 includes aradiation source 97 and a pinhole collimator 98. The pinhole collimator98 defines a scanning aperture that may be moved along the Z directionas seen in FIG. 10 to irradiate the sidewall 94 of the detector 93 atdifferent DOI's (different Z coordinates). In this way, events withknown DOI's and known photon energy are created having different lateralpositions, with the lateral positions depending on the absorptionstatistic of the irradiation via the sidewall. By measuring resultinginduced negative charges for different DOI's, the negative values of thetotal induced charge for non-collected adjacent signals may be used tocreate a look-up table or other relationship for deriving DOI frominduced non-collected charge.

It may further be noted that, since the negative value of the inducedcharge also depends on the energy of the absorbed photon, thecalibration may also account for photon energy. For example, the DOI maybe calibrated based on a ratio between a negative value of the inducednon-collected signal and the amplitude of the primary or collectedsignal. Such a ratio in various embodiments may be expressed as thefollowing:

${DOI} \propto \frac{V_{\lbrack{{negative}\mspace{14mu} {value}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {induced}\mspace{14mu} {charge}}\rbrack}}{V_{\lbrack{{amplitude}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {primary}\mspace{14mu} {signal}}\rbrack}}$

Since the negative value of the induced signal is independent (oressentially or substantially independent as discussed herein) of thelateral position (or X,Y coordinates), all of the adjacent orneighboring pixels will produce similar negative signals. Accordingly,signal-to-noise ration may be improved by adding the negative signalfrom a number of adjacent or neighboring pixels. Such a ratio in variousembodiments may be expressed as:

${DOI} \propto \frac{\sum\limits_{i = 1}^{i = N}{V\mspace{14mu}\lbrack {{negative}\mspace{14mu} {value}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {induced}\mspace{14mu} {charge}} \rbrack}_{i}}{V_{\lbrack{{amplitude}\mspace{14mu} {of}\mspace{14mu} {the}\mspace{14mu} {primary}\mspace{14mu} {signal}}\rbrack}}$

It may be noted that the negative induced charge or signal of anadjacent pixel and the primary signal in various embodiments aremeasured after shapers that are configured to shape the received oracquired signals. In various embodiments both signals have a generallystep-like shape and generally similar peaking and shaping times.Accordingly, the ratio between the signals may be about the same eitherafter the shapers or immediately after amplifiers from which the shapersreceive the signals.

FIG. 11 provides a schematic view of a radiation detector assembly 100in accordance with various embodiments. As seen in FIG. 11, theradiation detector assembly 100 includes a semiconductor detector 110and a processing unit 120. The semiconductor detector 110 has a surface112 on which plural pixelated anodes 114 are disposed. In the depictedembodiment, a cathode 142 is disposed on a surface opposite the surface112 on which the pixelated anodes 114 are disposed. For example, asingle cathode may be deposited on one surface with the pixelated anodesdisposed on an opposite surface. Generally, when radiation (e.g., one ormore photons) impacts the pixelated anodes 114, the semiconductordetector 110 generates electrical signals corresponding to the radiationpenetrating via the surface of cathode 142 and being absorbed in thevolume of detector 110 under surface 112. In the illustrated embodiment,the pixelated anodes 114 are shown in a 5×5 array for a total of 25pixelated anodes 114; however, it may be noted that other numbers orarrangements of pixelated anodes may be used in various embodiments.Each pixelated anode 114, for example, may have a surface area of 2.5millimeters square; however, other sizes and/or shapes may be employedin various embodiments.

The semiconductor detector 110 in various embodiments may be constructedusing different materials, such as semiconductor materials, includingCadmium Zinc Telluride (CdZnTe), often referred to as CZT, CadmiumTelluride (CdTe), and Silicon (Si), among others. The detector 110 maybe configured for use with, for example, nuclear medicine (NM) imagingsystems, positron emission tomography (PET) imaging systems, and/orsingle photon emission computed tomography (SPECT) imaging systems.

In the illustrated embodiment, each pixelated anode 114 generatesdifferent signals depending on the lateral location (e.g., in the X, Ydirections) of where a photon is absorbed in the volume of detector 110under the surface 112. For example, each pixelated anode 114 generates aprimary or collected signal responsive to the absorption of a photon inthe volume of detector 110 under the particular pixelated anode 114through which the photon penetrates into the detector volume. Thevolumes of detector 110 under pixelated anodes 114 are defined as voxels(not shown). For each pixelated anode 114, detector 110 has thecorresponding voxel. The absorption of a photon by a certain voxelcorresponding to a particular pixelated anode 114 a also results in aninduced charge that may be detected by pixels 114 b adjacent to orsurrounding the particular pixelated anode 114 a that receives thephoton. The charge detected by an adjacent or surrounding pixel may bereferred to herein as a non-collected charge, and result in anon-collected or secondary signal. A primary signal may includeinformation regarding photon energy (e.g., a distribution across a rangeof energy levels) as well as location information corresponding to theparticular pixelated anode 114 at which a photon penetrates via thesurface of cathode 142 and is absorbed in the corresponding voxel.

For example, in FIG. 1, a photon 116 is shown impacting the pixelatedanode 114 a to be absorbed in the corresponding voxel. Accordingly, thepixelated anode 114 a generates a primary signal responsive to receptionof the photon 116. As also seen in FIG. 1, pixelated anodes 114 b areadjacent to the pixelated anode 114 a. Pixelated anode 114 a has 8adjacent pixelated anodes 114 b. When the pixelated anode 114 a isimpacted by the photon 116, a charge is induced in and collected by thepixelated anode 114 a to produce the primary signal. One or more of theadjacent pixelated anodes 114 b generates a secondary signal responsiveto the induced charge generated in and collected by the pixelated anode114 a, which produces the primary signal. The secondary signal has anamplitude that is smaller than the primary signal. For any given photon,the corresponding primary signal (from the impacted pixel) and secondarysignals (from one or more pixels adjacent to the impacted pixel) may beused to locate the reception point of a photon at a particular locationwithin the pixel (e.g., to identify a particular sub-pixel locationwithin the pixel).

As seen in FIG. 11, sidewalls 140 extend along a depth 150 in the Zdirection between the surface 112 and the cathode 142. The locationalong the Z direction along the depth 150 of absorption where the photon116 is absorbed is the DOI for the corresponding event. As discussedherein, the negative induced non-collected charge on one or moreadjacent pixelated anodes 114 b is used in the illustrated embodiment todetermine the DOI for the event corresponding to the impact of thephoton 116.

Each pixelated anode 114 may have associated therewith one or moreelectronics channels configured to provide the primary and secondarysignals to one or more aspects of the processing unit 120 in cooperationwith the pixelated anodes. In some embodiments, all or a portion of eachelectronics channel may be disposed on the detector 110. Alternativelyor additionally, all or a portion of each electronics channel may behoused externally to the detector 110, for example as part of theprocessing unit 120, which may be or include an Application SpecificIntegration Circuit (ASIC). The electronics channels may be configuredto provide the primary and secondary signals to one or more aspects ofthe processing unit 120 while discarding other signals. For example, insome embodiments, each electronics channel includes a thresholddiscriminator. The threshold discriminator may allow signals exceeding athreshold level to be transmitted while preventing or inhibitingtransmission of signals that do not exceed a threshold level. Generally,the threshold level is set low enough to reliably capture the secondarysignals, while still being set high enough to exclude lower strengthsignals, for example due to noise. It may be noted that, because thesecondary signals may be relatively low in strength, the electronicsutilized are preferably low noise electronics to reduce or eliminatenoise that is not eliminated by the threshold level. In someembodiments, each electronic channel includes a peak-and-hold unit tostore electrical signal energy, and may also include a readoutmechanism. For example, the electronic channel may include arequest-acknowledge mechanism that allows the peak-and-hold energy andpixel location for each channel to be read out individually. Further, insome embodiments, the processing unit 120 or other processor may controlthe signal threshold level and the request-acknowledge mechanism.

In the illustrated embodiment, the processing unit 120 is operablycoupled to the pixelated anodes 114, and is configured to acquireprimary signals (for collected charges) and secondary signals (fornon-collected charges). For example, the processing unit 120 in variousembodiments acquires a primary signal from one of the anodes responsiveto reception of a photon by the anode. For example, a primary signal maybe acquired from pixelated anode 114 a responsive to reception of thephoton 116. The processing unit 120 also acquires at least one secondarysignal from at least one neighboring pixel (e.g., at least one adjacentanode 114 b) responsive to an induced charge caused by the reception ofthe photon. For example, a secondary signal may be acquired from one ormore of the adjacent pixels 114 b responsive to reception of the photon116. It may be noted that the secondary signal (or signals) and primarysignal generated responsive to reception of the photon 116 may beassociated with each other based on timing and location of detection ofthe corresponding charges.

The depicted processing unit 120 is also configured to determine a depthof interaction (DOI) in the semiconductor detector 110 for the receptionof the photon using (e.g., based on) the at least one secondary signal.For example, a DOI along the depth 150 where the photon 116 is absorbedmay be determined. In some embodiments, a total negative inducednon-collected charge for the at least one secondary signal may bedetermined, and used to determine the DOI as discussed herein. Invarious embodiments, a lookup table or other correlation may be used todetermine the DOI from a determined total negative induced non-collectedcharge for the at least one secondary signal. It may be noted that invarious embodiments, the processing unit 120 determines the DOI usingonly signals generated based on information from the pixelated anodes114, and without using any information from the cathode 142.Accordingly, construction and/or assembly of the detector assembly 100may be avoid or eliminate any hardware or electrical connections thatwould otherwise be necessary for acquiring signals from the cathode 142for use in determining DOI. Additionally, acquisitional and/orprocessing complexity or requirements may be further reduced by usingthe same information (primary and secondary signals) as discussed hereinto determine both DOI and sub-pixel location.

The determined DOI may be utilized to improve image quality. Forexample, the determined DOI may be used to correct or adjust acquiredimaging information. In some embodiments, the processing unit 120 isconfigured to adjust an energy level for an event corresponding to thereception of a photon by an anode based on the DOI. It may be noted thatcharge loss for a detected event depends on the distance of theabsorption for the event from the anode. Accordingly, the DOI for anumber of events may be used to adjust for charge loss to make theenergy levels for the events more uniform and/or closer to a photopeakfor accurate identification of events and accurate counting of events.

Alternatively or additionally, the processing unit 120 may be configuredto reconstruct an image using the DOI. For example, the DOI of a numberof events may be used directly by a reconstruction technique to utilize3D positioning of events in the detector for reconstruction. As anotherexample, the DOI may be used indirectly by a reconstruction techniquevia use of the DOI to correct energy levels, and then using thecorrected energy levels for image reconstruction.

As discussed herein, calibration information is utilized in variousembodiments. The processing unit 120 in various embodiments isconfigured to use calibration information (see, e.g., FIG. 10 andrelated discussion) to determine the DOI. The calibration may be in theform of a look up table or other relationship stored or otherwiseassociated with or accessible by the processing unit 120 (e.g., storedin memory 130). In some embodiments, the processing unit 120 isconfigured to determine the DOI using a calibration based on a ratiobetween a negative value of a single secondary signal and an amplitudeof the primary signal. (See FIG. 10 and related discussion.) As anotherexample, in some embodiments, the processing unit 120 is configured todetermine the DOI using a calibration based on a ratio between a sum orcombination of negative values for plural secondary signals (e.g.,signals from a number of adjacent pixels 114 b) and an amplitude of theprimary signal. (See FIG. 10 and related discussion.)

In various embodiments, the processing unit 120 may also be configuredto determine a sub-pixel location (e.g., a lateral location) for eventsusing the primary signal and at least one secondary signal in additionto determining the DOI. The sub-pixel location and the DOI may bedetermined using the same primary signal and at least one secondarysignal, providing for efficient determination of both. For example, thedepicted example processing unit 120 is configured to define sub-pixelsfor each pixelated anode. It may be noted that the sub-pixels in theillustrated embodiment (depicted as separated by dashed lines) are notphysically separate, but instead are virtual entities defined by theprocessing unit 120. Generally, the use of increasing numbers ofsub-pixels per pixel improves resolution while also increasingcomputational or processing requirements. The particular number ofsub-pixels defined or employed in a given application may be selectedbased on a balance between improved resolution against increasedprocessing requirements. In various embodiments, the use of virtualsub-pixels as discussed herein provides improved resolution whileavoiding or reducing costs associated with increasingly larger number ofincreasingly smaller pixelated anodes.

In the illustrated embodiment, the pixelated anode 114 a is shown asdivided into four sub-pixels, namely sub-pixel 150, sub-pixel 152,sub-pixel 154, and sub-pixel 156. While sub-pixels are shown in FIG. 11for only pixelated anode 114 a for clarity and ease of illustration, itmay be noted that the processing unit 120 in the illustrated embodimentalso defines corresponding sub-pixels for each of the remainingpixelated anodes 114. As seen in FIG. 11, the photon 116 is impacting aportion of the pixelated anode 114 a defined by the virtual sub-pixel150.

In the illustrated embodiment, the processing unit 120 acquires theprimary signal for a given acquisition event (e.g., impact of a photon)from the pixelated anode 114 a, along with timing (e.g., timestamp)information corresponding to a time of generation of the primary signaland location information identifying the pixelated anode 114 a as thepixelated anode corresponding to the primary signal. For example, anacquisition event such as a photon impacting a pixelated anode 114 mayresult in a number of counts occurring across a range or spectrum ofenergies, with the primary signal including information describing thedistribution of counts across the range or spectrum of energies. Theprocessing unit 120 also acquires one or more secondary signals for theacquisition event from the pixelated anodes 114 b, along with timestampinformation and location information for the secondary signal(s). Theprocessing unit 120 then determines the location for the givenacquisition event identifying the pixelated anode 114 a as the impactedpixelated anode 114 a, and then determining which of sub-pixels 150,152, 154, 156 define the location of impact for the acquisition event.Using conventional methods, the location of sub-pixels 150, 152, 154,156 may be derived based on the location (e.g., associated pixelatedanode) and the relationships between the strengths of the primary signalin the associated pixelated anode 114 a and the secondary signal(s) inthe adjacent pixelated anodes 114 b for the acquisition event. Theprocessing unit 120 may use time stamp information as well as locationinformation to associate the primary signal and secondary signalsgenerated responsive to the given acquisition event with each other, andto discriminate the primary signal and secondary signals for the givenacquisition event from signals for other acquisition events occurringduring a collection or acquisition period using the time stamp andlocation information. Accordingly, the use of time stamp informationhelps allow for distinguishing between the primary signal and itscorresponding secondary signals from random coincidence that may occurbetween primary signals of adjacent pixels, since the timestamps fromthe primary signal and its corresponding secondary signals arecorrelated for a particular acquisition event.

Additional discussion regarding virtual sub-pixels and the use ofvirtual sub-pixels, and the use of collected and non-collected chargesignals may be found in U.S. Patent Application Serial No. 14/724,022,entitled “Systems and Method for Charge-Sharing Identification andCorrection Using a Single Pixel,” filed 28 May 2015 (“the 022Application); U.S. patent application Ser. No. 15/280,640, entitled“Systems and Methods for Sub-Pixel Location Determination,” filed 29Sep. 2016 (“the 640 Application”); and U.S. patent application Ser. No.14/627,436, entitled “Systems and Methods for Improving EnergyResolution by Sub-Pixel Energy Calibration,” filed 20 Feb. 2015 (“the436 Application). The subject matter of each of the 022 Application, the640 Application, and the 436 Application are incorporated by referencein its entirety.

In various embodiments the processing unit 120 includes processingcircuitry configured to perform one or more tasks, functions, or stepsdiscussed herein. It may be noted that “processing unit” as used hereinis not intended to necessarily be limited to a single processor orcomputer. For example, the processing unit 120 may include multipleprocessors, ASIC's, FPGA's, and/or computers, which may be integrated ina common housing or unit, or which may distributed among various unitsor housings. It may be noted that operations performed by the processingunit 120 (e.g., operations corresponding to process flows or methodsdiscussed herein, or aspects thereof) may be sufficiently complex thatthe operations may not be performed by a human being within a reasonabletime period. For example, the determination of values of collected andnon-collected charges, and/or the determination of DOI's and/orsub-pixel locations based on the collected and/or non-collected chargeswithin the time constraints associated with such signals may rely on orutilize computations that may not be completed by a person within areasonable time period.

The depicted processing unit 120 includes a memory 130. The memory 130may include one or more computer readable storage media. The memory 130,for example, may store mapping information describing the sub-pixellocations, acquired emission information, image data corresponding toimages generated, results of intermediate processing steps, calibrationparameters or calibration information (e.g., a lookup table correlatingnegative induced charge value to DOI), or the like. Further, the processflows and/or flowcharts discussed herein (or aspects thereof) mayrepresent one or more sets of instructions that are stored in the memory130 for direction of operations of the radiation detection assembly 100.

FIG. 12 provides a flowchart of a method 200 (e.g., for determiningDOI), in accordance with various embodiments. The method 200, forexample, may employ or be performed by structures or aspects of variousembodiments (e.g., systems and/or methods and/or process flows)discussed herein. In various embodiments, certain steps may be omittedor added, certain steps may be combined, certain steps may be performedconcurrently, certain steps may be split into multiple steps, certainsteps may be performed in a different order, or certain steps or seriesof steps may be re-performed in an iterative fashion. In variousembodiments, portions, aspects, and/or variations of the method 200 maybe able to be used as one or more algorithms to direct hardware (e.g.,one or more aspects of the processing unit 120) to perform one or moreoperations described herein.

At 202, primary signals and secondary signals are acquired correspondingto acquisition events (e.g., events corresponding to reception ofphotons). The primary and secondary signals are generated responsive toreception of photons by a semiconductor detector, and are received frompixelated anodes (e.g., anodes of a semiconductor device of an imagingsystem such as assembly 100). For example, a patient that has beenadministered at least one radiopharmaceutical may be placed within afield of view of one or more detectors, and radiation (e.g., photons)emitted from the patient may impact the pixelated anodes disposed onreception surfaces of the one or more detectors resulting in acquisitionevents (e.g., photon impacts). For a given photon impact in the depictedexample embodiment, a primary signal (responsive to a collected charge)is generated by the impacted pixelated anode (or collecting anode), andone or more secondary signals (responsive to non-collected charge) aregenerated by pixelated anodes adjacent to the impacted pixelated anode(or non-collecting anodes).

At 204, a depth of interaction (DOI) in the semiconductor device isdetermined for the acquisition events resulting in the primary andsecondary signals acquired at 202. In various embodiments, the DOI for agiven event is determined using at least one secondary signal for thatparticular event. For example, as discussed herein, the DOI in variousembodiments is determined based on a total negative inducednon-collected charge value from one or more adjacent or non-collectingpixels (e.g., at least one adjacent pixelated anode). It may be notedthat the DOI in various embodiments is determined without using anyinformation (e.g., detected charge or corresponding signals) from acathode of the detector assembly.

In various embodiments, the total negative non-collecting induced chargemay be adjusted or corrected to account for variations in semiconductorconstruction and/or photon energies. For example, in the depictedembodiment, at 206, calibration information is used to determine theDOI. As discussed herein, in some embodiments, the DOI may be determinedusing a calibration based on a ratio between a negative value of asingle secondary signal and an amplitude of the primary signal, and insome embodiments, the DOI may be determined using a calibration based ona ratio between a sum or combination of negative values for pluralsecondary signals and an amplitude of the primary signal. (See FIG. 10and related discussion.)

At 208, a sub-pixel location is determined using the primary signal andthe at least one secondary signal. For each event, a correspondingsub-pixel location may be determined. It may be noted that the sameinformation (primary and secondary signals) used to determine DOI's forevents may also be used to determine sub-pixel locations for thoseevents.

At 210, an energy level for an event is adjusted based on the DOI. Forexample, as the energy detected may vary based on DOI, the DOI for eachacquired event may be used to adjust the corresponding energy levelsbased on the corresponding DOI's to make the energy levels for a groupof events more consistent and/or closer to a target or otherpredetermined energy level.

At 212, an image is reconstructed using the DOI. For example, correctedenergy levels from 210 may be used in the reconstruction of an image. Asanother example, the DOIs for events may be used to determine 3Dpositioning information of those events within a detector, with the 3Dpositioning information used to reconstruct an image.

As discussed herein, a radiation detector system (e.g., a systemconfigured to determine DOI using secondary signals corresponding tonon-collected induced charges) may be calibrated. FIG. 13 provides aflowchart of a method 300 (e.g., for providing and calibrating aradiation detector assembly), in accordance with various embodiments.The method 300, for example, may employ or be performed by structures oraspects of various embodiments (e.g., systems and/or methods and/orprocess flows) discussed herein. In various embodiments, certain stepsmay be omitted or added, certain steps may be combined, certain stepsmay be performed concurrently, certain steps may be split into multiplesteps, certain steps may be performed in a different order, or certainsteps or series of steps may be re-performed in an iterative fashion. Invarious embodiments, portions, aspects, and/or variations of the method300 may be able to be used as one or more algorithms to direct hardware(e.g., one or more aspects of the processing unit 120) to perform one ormore operations described herein.

At 302, a semiconductor detector (e.g., semiconductor detector 110 ofradiation imaging assembly 100) is provided. The semiconductor detectorof the illustrated example has a surface with plural pixelated anodesdisposed on the surface. Each pixelated anode is configured to generatea primary signal responsive to reception of a photon by the pixelatedanode and to generate at least one secondary signal responsive to aninduced charge caused by reception of a photon by at least one adjacentanode. At 304, the pixelated anodes are operably coupled to at least oneprocessor (e.g., processing unit 120).

At 306, a calibrated radiation supply (e.g., having a known photonenergy) is provided at different depths along a sidewall of thesemiconductor detector. Responsive to reception of the calibratedradiation supply, the pixelated anodes generate primary and secondarysignals. For example, the calibrated radiation supply may be passedthrough a pin-hole collimator to the sidewall of the semiconductordetector. The location of a given pin-hole (e.g., in a Z direction)through which radiation is passed may be used to determine the DOI atwhich the corresponding radiation is passed from the collimator andreceived by the semiconductor detector. At 308, the primary andsecondary signals are acquired from the pixelated anodes by the at leastone processor.

At 310, corresponding negative values of total induced charges for eachof the different depths at which the radiation has been supplied aredetermined. At 312, calibration information (e.g., a look-up table orother correlating relationship between DOI's and negative inducednon-collected charge values) is determined.

FIG. 14 is a schematic illustration of a NM imaging system 1000 having aplurality of imaging detector head assemblies mounted on a gantry (whichmay be mounted, for example, in rows, in an iris shape, or otherconfigurations, such as a configuration in which the movable detectorcarriers 1016 are aligned radially toward the patient-body 1010). Inparticular, a plurality of imaging detectors 1002 are mounted to agantry 1004. In the illustrated embodiment, the imaging detectors 1002are configured as two separate detector arrays 1006 and 1008 coupled tothe gantry 1004 above and below a subject 1010 (e.g., a patient), asviewed in FIG. 14. The detector arrays 1006 and 1008 may be coupleddirectly to the gantry 1004, or may be coupled via support members 1012to the gantry 1004 to allow movement of the entire arrays 1006 and/or1008 relative to the gantry 1004 (e.g., transverse translating movementin the left or right direction as viewed by arrow T in FIG. 14).Additionally, each of the imaging detectors 1002 includes a detectorunit 1014, at least some of which are mounted to a movable detectorcarrier 1016 (e.g., a support arm or actuator that may be driven by amotor to cause movement thereof) that extends from the gantry 1004. Insome embodiments, the detector carriers 1016 allow movement of thedetector units 1014 towards and away from the subject 1010, such aslinearly. Thus, in the illustrated embodiment the detector arrays 1006and 1008 are mounted in parallel above and below the subject 1010 andallow linear movement of the detector units 1014 in one direction(indicated by the arrow L), illustrated as perpendicular to the supportmember 1012 (that are coupled generally horizontally on the gantry1004). However, other configurations and orientations are possible asdescribed herein. It should be noted that the movable detector carrier1016 may be any type of support that allows movement of the detectorunits 1014 relative to the support member 1012 and/or gantry 1004, whichin various embodiments allows the detector units 1014 to move linearlytowards and away from the support member 1012.

Each of the imaging detectors 1002 in various embodiments is smallerthan a conventional whole body or general purpose imaging detector. Aconventional imaging detector may be large enough to image most or allof a width of a patient's body at one time and may have a diameter or alarger dimension of approximately 50 cm or more. In contrast, each ofthe imaging detectors 1002 may include one or more detector units 1014coupled to a respective detector carrier 1016 and having dimensions of,for example, 4 cm to 20 cm and may be formed of Cadmium Zinc Telluride(CZT) tiles or modules. For example, each of the detector units 1014 maybe 8×8 cm in size and be composed of a plurality of CZT pixelatedmodules (not shown). For example, each module may be 4×4 cm in size andhave 16×16=256 pixels. In some embodiments, each detector unit 1014includes a plurality of modules, such as an array of 1×7 modules.However, different configurations and array sizes are contemplatedincluding, for example, detector units 1014 having multiple rows ofmodules.

It should be understood that the imaging detectors 1002 may be differentsizes and/or shapes with respect to each other, such as square,rectangular, circular or other shape. An actual field of view (FOV) ofeach of the imaging detectors 1002 may be directly proportional to thesize and shape of the respective imaging detector.

The gantry 1004 may be formed with an aperture 1018 (e.g., opening orbore) therethrough as illustrated. A patient table 1020, such as apatient bed, is configured with a support mechanism (not shown) tosupport and carry the subject 1010 in one or more of a plurality ofviewing positions within the aperture 1018 and relative to the imagingdetectors 1002. Alternatively, the gantry 1004 may comprise a pluralityof gantry segments (not shown), each of which may independently move asupport member 1012 or one or more of the imaging detectors 1002.

The gantry 1004 may also be configured in other shapes, such as a “C”,“H” and “L”, for example, and may be rotatable about the subject 1010.For example, the gantry 1004 may be formed as a closed ring or circle,or as an open arc or arch which allows the subject 1010 to be easilyaccessed while imaging and facilitates loading and unloading of thesubject 1010, as well as reducing claustrophobia in some subjects 1010.

Additional imaging detectors (not shown) may be positioned to form rowsof detector arrays or an arc or ring around the subject 1010. Bypositioning multiple imaging detectors 1002 at multiple positions withrespect to the subject 1010, such as along an imaging axis (e.g., headto toe direction of the subject 1010) image data specific for a largerFOV may be acquired more quickly.

Each of the imaging detectors 1002 has a radiation detection face, whichis directed towards the subject 1010 or a region of interest within thesubject.

In various embodiments, multi-bore collimators may be constructed to beregistered with pixels of the detector units 1014, which in oneembodiment are CZT detectors. However, other materials may be used.Registered collimation may improve spatial resolution by forcing photonsgoing through one bore to be collected primarily by one pixel.Additionally, registered collimation may improve sensitivity and energyresponse of pixelated detectors as detector area near the edges of apixel or in-between two adjacent pixels may have reduced sensitivity ordecreased energy resolution or other performance degradation. Havingcollimator septa directly above the edges of pixels reduces the chanceof a photon impinging at these degraded-performance locations, withoutdecreasing the overall probability of a photon passing through thecollimator.

A controller unit 1030 may control the movement and positioning of thepatient table 1020, imaging detectors 1002 (which may be configured asone or more arms), gantry 1004 and/or the collimators 1022 (that movewith the imaging detectors 1002 in various embodiments, being coupledthereto). A range of motion before or during an acquisition, or betweendifferent image acquisitions, is set to maintain the actual FOV of eachof the imaging detectors 1002 directed, for example, towards or “aimedat” a particular area or region of the subject 1010 or along the entiresubject 1010. The motion may be a combined or complex motion in multipledirections simultaneously, concurrently, or sequentially as described inmore detail herein.

The controller unit 1030 may have a gantry motor controller 1032, tablecontroller 1034, detector controller 1036, pivot controller 1038, andcollimator controller 1040. The controllers 1030, 1032, 1034, 1036,1038, 1040 may be automatically commanded by a processing unit 1050,manually controlled by an operator, or a combination thereof. The gantrymotor controller 1032 may move the imaging detectors 1002 with respectto the subject 1010, for example, individually, in segments or subsets,or simultaneously in a fixed relationship to one another. For example,in some embodiments, the gantry controller 1032 may cause the imagingdetectors 1002 and/or support members 1012 to move relative to or rotateabout the subject 1010, which may include motion of less than or up to180 degrees (or more).

The table controller 1034 may move the patient table 1020 to positionthe subject 1010 relative to the imaging detectors 1002. The patienttable 1020 may be moved in up-down directions, in-out directions, andright-left directions, for example. The detector controller 1036 maycontrol movement of each of the imaging detectors 1002 to move togetheras a group or individually as described in more detail herein. Thedetector controller 1036 also may control movement of the imagingdetectors 1002 in some embodiments to move closer to and farther from asurface of the subject 1010, such as by controlling translating movementof the detector carriers 1016 linearly towards or away from the subject1010 (e.g., sliding or telescoping movement). Optionally, the detectorcontroller 1036 may control movement of the detector carriers 1016 toallow movement of the detector array 1006 or 1008. For example, thedetector controller 1036 may control lateral movement of the detectorcarriers 1016 illustrated by the T arrow (and shown as left and right asviewed in FIG. 14). In various embodiments, the detector controller 1036may control the detector carriers 1016 or the support members 1012 tomove in different lateral directions. Detector controller 1036 maycontrol the swiveling motion of detectors 1002 together with theircollimators 1022.

The pivot controller 1038 may control pivoting or rotating movement ofthe detector units 1014 at ends of the detector carriers 1016 and/orpivoting or rotating movement of the detector carrier 1016. For example,one or more of the detector units 1014 or detector carriers 1016 may berotated about at least one axis to view the subject 1010 from aplurality of angular orientations to acquire, for example, 3D image datain a 3D SPECT or 3D imaging mode of operation. The collimator controller1040 may adjust a position of an adjustable collimator, such as acollimator with adjustable strips (or vanes) or adjustable pinhole(s).

It should be noted that motion of one or more imaging detectors 1002 maybe in directions other than strictly axially or radially, and motions inseveral motion directions may be used in various embodiment. Therefore,the term “motion controller” may be used to indicate a collective namefor all motion controllers. It should be noted that the variouscontrollers may be combined, for example, the detector controller 1036and pivot controller 1038 may be combined to provide the differentmovements described herein.

Prior to acquiring an image of the subject 1010 or a portion of thesubject 1010, the imaging detectors 1002, gantry 1004, patient table1020 and/or collimators 1022 may be adjusted, such as to first orinitial imaging positions, as well as subsequent imaging positions. Theimaging detectors 1002 may each be positioned to image a portion of thesubject 1010. Alternatively, for example in a case of a small sizesubject 1010, one or more of the imaging detectors 1002 may not be usedto acquire data, such as the imaging detectors 1002 at ends of thedetector arrays 1006 and 1008, which as illustrated in FIG. 14 are in aretracted position away from the subject 1010. Positioning may beaccomplished manually by the operator and/or automatically, which mayinclude using, for example, image information such as other imagesacquired before the current acquisition, such as by another imagingmodality such as X-ray Computed Tomography (CT), MM, X-Ray, PET orultrasound. In some embodiments, the additional information forpositioning, such as the other images, may be acquired by the samesystem, such as in a hybrid system (e.g., a SPECT/CT system).Additionally, the detector units 1014 may be configured to acquirenon-NM data, such as x-ray CT data. In some embodiments, amulti-modality imaging system may be provided, for example, to allowperforming NM or SPECT imaging, as well as x-ray CT imaging, which mayinclude a dual-modality or gantry design as described in more detailherein.

After the imaging detectors 1002, gantry 1004, patient table 1020,and/or collimators 1022 are positioned, one or more images, such asthree-dimensional (3D) SPECT images are acquired using one or more ofthe imaging detectors 1002, which may include using a combined motionthat reduces or minimizes spacing between detector units 1014. The imagedata acquired by each imaging detector 1002 may be combined andreconstructed into a composite image or 3D images in variousembodiments.

In one embodiment, at least one of detector arrays 1006 and/or 1008,gantry 1004, patient table 1020, and/or collimators 1022 are moved afterbeing initially positioned, which includes individual movement of one ormore of the detector units 1014 (e.g., combined lateral and pivotingmovement) together with the swiveling motion of detectors 1002. Forexample, at least one of detector arrays 1006 and/or 1008 may be movedlaterally while pivoted. Thus, in various embodiments, a plurality ofsmall sized detectors, such as the detector units 1014 may be used for3D imaging, such as when moving or sweeping the detector units 1014 incombination with other movements.

In various embodiments, a data acquisition system (DAS) 1060 receiveselectrical signal data produced by the imaging detectors 1002 andconverts this data into digital signals for subsequent processing.However, in various embodiments, digital signals are generated by theimaging detectors 1002. An image reconstruction device 1062 (which maybe a processing device or computer) and a data storage device 1064 maybe provided in addition to the processing unit 1050. It should be notedthat one or more functions related to one or more of data acquisition,motion control, data processing and image reconstruction may beaccomplished through hardware, software and/or by shared processingresources, which may be located within or near the imaging system 1000,or may be located remotely. Additionally, a user input device 1066 maybe provided to receive user inputs (e.g., control commands), as well asa display 1068 for displaying images. DAS 1060 receives the acquiredimages from detectors 1002 together with the corresponding lateral,vertical, rotational and swiveling coordinates of gantry 1004, supportmembers 1012, detector units 1014, detector carriers 1016, and detectors1002 for accurate reconstruction of an image including 3D images andtheir slices.

It may be noted that the embodiment of FIG. 14 may be understood as alinear arrangement of detector heads (e.g., utilizing detector unitsarranged in a row and extending parallel to one another. In otherembodiments, a radial design may be employed. Radial designs, forexample, may provide additional advantages in terms of efficientlyimaging smaller objects, such as limbs, heads, or infants. FIG. 15provides a schematic view of a nuclear medicine (NM) multi-head imagingsystem 1100 in accordance with various embodiments. Generally, theimaging system 1100 is configured to acquire imaging information (e.g.,photon counts) from an object to be imaged (e.g., a human patient) thathas been administered a radiopharmaceutical. The depicted imaging system1100 includes a gantry 1110 having a bore 1112 therethrough, pluralradiation detector head assemblies 1115, and a processing unit 1120.

The gantry 1110 defines the bore 1112. The bore 1112 is configured toaccept an object to be imaged (e.g., a human patient or portionthereof). As seen in FIG. 15, plural radiation detector head assemblies1115 are mounted to the gantry 1110. In the illustrated embodiment, eachradiation detector head assembly 1115 includes an arm 1114 and a head1116. The arm 1114 is configured to articulate the head 1116 radiallytoward and/or away from a center of the bore 1112 (and/or in otherdirections), and the head 1116 includes at least one detector, with thehead 1116 disposed at a radially inward end of the arm 1114 andconfigured to pivot to provide a range of positions from which imaginginformation is acquired.

The detector of the head 1116, for example, may be a semiconductordetector. For example, a semiconductor detector various embodiments maybe constructed using different materials, such as semiconductormaterials, including Cadmium Zinc Telluride (CdZnTe), often referred toas CZT, Cadmium Telluride (CdTe), and Silicon (Si), among others. Thedetector may be configured for use with, for example, nuclear medicine(NM) imaging systems, positron emission tomography (PET) imagingsystems, and/or single photon emission computed tomography (SPECT)imaging systems.

In various embodiments, the detector may include an array of pixelatedanodes, and may generate different signals depending on the location ofwhere a photon is absorbed in the volume of the detector under a surfaceif the detector. The volumes of the detector under the pixelated anodesare defined as voxels. For each pixelated anode, the detector has acorresponding voxel. The absorption of photons by certain voxelscorresponding to particular pixelated anodes results in chargesgenerated that may be counted. The counts may be correlated toparticular locations and used to reconstruct an image.

In various embodiments, each detector head assembly 1115 may define acorresponding view that is oriented toward the center of the bore 1112.Each detector head assembly 1115 in the illustrated embodiment isconfigured to acquire imaging information over a sweep rangecorresponding to the view of the given detector unit. Additional detailsregarding examples of systems with detector units disposed radiallyaround a bore may be found in U.S. patent application Ser. No.14/788,180, filed 30 Jun. 2015, entitled “Systems and Methods ForDynamic Scanning With Multi-Head Camera,” the subject matter of which isincorporated by reference in its entirety.

The processing unit 1120 includes memory 1122. The imaging system 1100is shown as including a single processing unit 1120; however, the blockfor the processing unit 1120 may be understood as representing one ormore processors that may be distributed or remote from each other. Thedepicted processing unit 1120 includes processing circuitry configuredto perform one or more tasks, functions, or steps discussed herein. Itmay be noted that “processing unit” as used herein is not intended tonecessarily be limited to a single processor or computer. For example,the processing unit 1120 may include multiple processors and/orcomputers, which may be integrated in a common housing or unit, or whichmay distributed among various units or housings.

Generally, various aspects (e.g., programmed modules) of the processingunit 1120 act individually or cooperatively with other aspects toperform one or more aspects of the methods, steps, or processesdiscussed herein. In the depicted embodiment, the memory 1122 includes atangible, non-transitory computer readable medium having stored thereoninstructions for performing one or more aspects of the methods, steps,or processes discussed herein.

It may be noted that the energy resolution of various radiationdetectors, such as detectors made of CZT, and the low energy-tail oftheir spectrum are affected by the Depth of Interaction (DOI) at whichthe the photons of the detected radiation are absorbed in the detector.For example, generally, the deeper the photon is absorbed in thedetector (e.g., a relatively large value for the DOI, or farther awayfrom the cathode), the lower is the electrical charge (or magnitude ofan electrical signal representing the photon energy) produced by thedetector. Since the photons of the detected radiation are absorbed at avariety of DOI's, a corresponding variety of signals with differentintensities (energies) will be produced.

It may be noted that the peak energy of the signals is produced when thephotons are absorbed very close to the cathode of the detector. Photonsthat are absorbed relatively deeper but still close to the cathodeproduce signals with slightly lower energy than corresponding photonswith the same energy that are absorbed closer to the cathode. Thesevarying energies broaden the energy peak of the spectrum and can resultin degraded energy-resolution. When photons with the same energy of thedetected radiation are absorbed much deeper in the detector, theresulting energy signals may be significantly lower than the peak energyof the detector, and tend to increase the number of events that appearin the low-energy tail of the detector spectrum, resulting in arelatively higher low-energy tail, and resulting in detector sensitivitydegradation. Generally, wide energy peaks and high low-energy tail arenot desirable in the detector spectrum. These undesired phenomena maycorrected using the conventionally known Hecht equation, which providesanalytical mathematical relations between the DOI and the detectorsignal. However, such analytical approaches are correct only fordetectors having uniform internal electrical fields and anodes havingdimensions that are similar to the detector thickness (e.g., no smallpixel effect). When the detector is affected by small-pixel effect, acorrection may be applied to Hecht equation. However, when the internalelectrical field of the detector is non-uniform, use of the Hechtequation may provide insufficient accuracy.

Accordingly, various embodiments provide systems and/or methods thatempirically provide relationships (and/or utilize empiricalrelationships) between the DOI of a detector and the electrical signalsthat it produces, in a primary pixel, to allow energy-resolution andlow-energy tail corrections based on the negative values of one or moresecondary signals induced in pixels surrounding the primary pixel.Various embodiments provide systems and/or methods that empiricallyprovide the relations (and/or utilize such relationships) between thenegative induced signals developed in one or more pixels that areadjacent to the primary pixel that measures an event and the primaryelectrical signals to allow energy-resolution and low-energy tailcorrections. Further, various embodiments provide systems and/or methodsto correct the energy-resolution and low-energy tail of a detectorspectrum using the negative values of the induced signals developed byone or more pixels that are adjacent to the primary pixel that producesthe detector spectrum.

For example, FIG. 16A provides a schematic block view of a radiationdetector assembly 1200 that develops and/or utilizes negative values ofinduced or secondary signals to correct energy levels and provideimproved energy resolution and/or sensitivity. It may be noted that theradiation detector assembly 1200 depicted in FIG. 16 may be generallysimilar in various respects to the radiation detector assembly 100discussed in connection with FIG. 11, and/or incorporate aspects of theradiation detector assembly 100. The depicted radiation detectorassembly 1200 has a nonuniform internal electrical field.

It may be noted that for radiation detectors such as radiation detectorassembly 1200 having nonuniform internal electrical fields (either withor without the small-pixel effect), the relationship between the DOI andthe energy signal E produced by the detector, in a primary pixel, may begiven by:

E=f(DOI)  Eq(1)

It may be noted that the function f is not an analytical function (e.g.,due to the nonuniform internal electrical field). As discussed above inconnection with FIGS. 1-15, the negative value of the induced signal S,in a secondary pixel, depends on the DOI and may be given by anothernon-analytical function g as follows:

S=g(DOI)  Eq (2)

Accordingly, the DOI may be expressed as follows:

DOI=g ⁻¹(S)  Eq(3)

Where g⁻¹ is the inverse function of g. Accordingly, the energy signal Emay also expressed as:

E=f(DOI)=f(g ⁻¹(S))  Eq(4)

Put another way:

E=F(S)  Eq(5)

Where:

F(S)=f(g ⁻¹(S))  Eq(6)

It may be noted that F(S) is not an analytical function. Varioustechniques to derive F(S) empirically for correcting theenergy-resolution and the low-energy tail are discussed below.

FIG. 16A schematically illustrates the radiation detector assembly 1200,and FIGS. 16B and 16C provide graphs 1242 and 1250, respectively, whichshow the signals output by the radiation detector assembly 1200 from itselectronic channels 1209 and 1211 that are electrically connected to aprimary pixel 1210 and a secondary pixel 1208, respectively. In theillustrated example, the secondary pixel 1208 is adjacent to the primarypixel 1210, with the primary pixel 1210 at a location corresponding tophoton absorption in the radiation detector assembly 1200 as furtherdiscussed below.

The radiation detector assembly 1200 includes a semiconductor 1202(e.g., a CZT semiconductor plate). The semiconductor 1202 has a surface1203. The depicted radiation detector assembly 1200 also includes amonolithic cathode 1204, and pixilated anodes 1206, 1208, 1210. Theanodes 1206, 1208, 1210 are electrically coupled to electronic channels1209, 1211, and 1213, respectively, with the pixelated anodes 1206,1208, 1210 disposed on the surface 1203, opposite the cathode 1204. Theradiation detector assembly 1200 has a thickness 1230 that is equal toD. As seen in FIG. 16A, the depicted radiation detector assembly 1200receives radiation including photons 1236 via the cathode 1204. Thephotons 1236 are absorbed in the radiation detector assembly 1200 atvarious DOI's measured along a Z coordinate illustrated by arrows 1232in FIG. 16A. The lateral coordinate X is illustrated by arrow 1234.Pixels 1210 and 1208 define or correspond to voxels 1222 and 1220,respectively. The radiation detector assembly 1200 also includes aprocessing unit 1290 and memory 1292. The processing unit 1290, forexample, receives signals from the electronic channels 1209, 1211, and1213, and performs operations utilizing the signals as discussed herein.It may be noted that the processing unit 1290 may be generally similarto the processing unit 120, and/or incorporate one or more aspects ofthe processing unit 120. (For example, the processing unit 1290 mayinclude processing circuitry configured to perform one or more tasks,functions, or steps discussed herein. Further, it may again be notedthat “processing unit” as used herein is not intended to necessarily belimited to a single processor or computer. For example, the processingunit 1290 may include multiple processors, ASIC's, FPGA's, and/orcomputers, which may be integrated in a common housing or unit, or whichmay distributed among various units or housings.)

The processing unit 1290 in various embodiments is operably coupled tothe pixelated anodes 1206, 1208, 1210, and is configured (e.g.,programmed) to acquire a primary signal from one of the anodes (e.g.,pixel 1210 in the illustrated example) responsive to reception of aphoton by the anode, and acquire at least one secondary signal from atleast one neighboring pixel (e.g., pixel 1208 in the illustratedexample). As discussed herein, the at least one secondary signal definesa negative value. The depicted processing unit 1290 is furtherconfigured to determine an energy correction factor for the reception ofthe phyon using the negative value of the at least one secondary signal.For example, as discussed in greater detail below, the processing unit1290 may use a relationship of a value of the primary signal with thenegative value of the at least one secondary signal to determe theenergy correction factor. In some embodiments, the processing unit 1290uses a plot of the negative value of the at least one secondary signalagainst the value of the primary signal. In some embodiments, the energycorrection factor is defined by a ratio between a magnitude of theprimary signal and a magnitude of the negative value. It may be notedthat, as used herein, reference to the expression “negative value” maybe used to describe the negative value of the at least one secondarysignal

Further, in various embodiments, the processing unit 1290 is configuredto use the energy correction factor to adjust an energy level ofdetected events (e.g., multiplying a detected energy level by thecorrection factor to adjust the energy level toward or to a target levelsuch as a maximum peak energy level). The processing unit 1290 may applythe energy correction factor to move at least one detected event fromoutside of an energy window used for counting (e.g., from a low energytail located outside of the energy window) to inside the energy windowused for counting, to increase the number of counts without requiringadditional scanning time. In various embodiments, the processing unit1290 is configured to organize detected events into groups based onmagnitudes of corresponding negative values, and to apply correspondingenergy correction factors to the groups. For example, the processingunit 1290 may be configured to define sub-spectra based on magnitudes ofcorresponding negative values for detected events, and align peaks ofthe sub-spectra using corresponding energy correction factors (e.g., adifferent energy correction factor for each group based on eachindividual group's negative value of the secondary signal). Additionaldetails regarding the determination and/or application of energycorrection factors are discussed below.

Generally, the photons 1236 of the illustrated example are absorbed inradiation detector assembly 1200 at various DOI's for correspondingdetection events. Each of the events occurs in the voxel 1222 defined byor corresponding to the primary pixel 1210. For example, DOI₀ for event1228 is located at Z=0 and is marked as Z₀, DOI₁ for event 1226 islocated at Z=Z₁ and is marked as Z₁, and DOI₂ for event 1224 is locatedat Z=Z₂ and is marked as Z₂. Photons 1236 absorbed at respective depthDOI₀, DOI₁ and DOI₂ produce corresponding events 1228, 1226, and 1224shown on lines 1218, 1216, and 1214, respectively. The signals producedby events 1228, 1226 and 1224, in electronic channel 1209 by primarypixel 1210, and in electronic channel 1211 by secondary pixel 1208 areshown by graph 1242 (FIG. 16B) and graph 1250 (FIG. 16C).

As seen in FIG. 16B, graph 1242 schematically shows the primary signalsof the charge Q (proportional to the energy signal E) induced andcollected by the primary pixel 1210 as a function of the coordinate Zalong which the charges of events 1228, 1226 and 1224 drift in voxel1222 toward the anode of primary pixel 1210 for collection by theprimary pixel 1210. Graph 1242 shows the induced and collected charge onthe primary anode or primary pixel 1210 produced by events 1228, 1226,and 1224 absorbed, respectively, at 1218 (Z=0), 1216 (Z=Z₁), and 1214(Z=Z₂). The charges produced by events 1228, 1226, and 1224 created at(Z=Z₀), (Z=Z₁), and (Z=Z₂) as they drift toward the primary anode orprimary pixel 1210 along coordinate Z are shown by curves 1244, 1246,and 1248 and marked by Q(Z=Z₀, Z), Q(Z=Z₁, Z), and Q(Z=Z₂, Z). Thecharges produced by events 1228, 1226, and 1224 when their chargesarrive to the primary pixel 1210 (Z=D) are marked by Q (Z=Z₀, D), Q(Z=Z₁, D) and Q (Z=Z₂, D). As seen in FIG. 16B, the primary signal (Q orE) depends on the DOI, such that it is different for each different DOI.The maximum signal Q or E occurs when the DOI=0. In such a case, theenergy of the primary signal measured by pixel 1210 is maximal and isequal to the peak-energy E_(P) in the spectrum of pixel 1210 of theradiation detector assembly 1200. It may be noted that where referenceis made to the primary induced and collected charge Q, it may bereplaced by the primary energy signal E since Q and E are proportionalto each other. Accordingly, it may be noted that references to charge Qare also generally similar to references to energy E, and vice versa.

As seen in FIG. 16C, graph 1250 schematically shows the secondaryinduced signals of the charge Q (proportional to the energy of thesecondary induced signal E or S), induced on the secondary anode orsecondary pixel 1208 that is adjacent to primary pixel 1210, as afunction of the coordinate Z, along which the charges of events 1228,1226 and 1224 in primary voxel 1222 drift toward the primary pixel 1210for collection. Graph 1250 shows the induced charge on the secondarypixel 1208 produced by events 1228, 1226, and 1224 generated at 1218(Z=0), 1216 (Z=Z₁) and 1214 (Z=Z₂), respectively. The non-collectedinduced charges produced by events 1228, 1226 and 1224, on the secondarypixel 1208, as the events are generated at (Z=Z₀), (Z=Z₁) and (Z=Z₂) anddrift toward the anode of pixel 1210, along coordinate Z, are shown bycurves 1252, 1254, and 1256, and marked by Q(Z=Z₀, Z), Q(Z=Z₁, Z) andQ(Z=Z₂, Z). The negative charges produced by events 1228, 1226 and 1224,on the secondary anode of secondary pixel 1208, when their chargesarrive at the primary pixel 1210 (Z=D) and are collected at the primarypixel 1210, are marked by Q (Z=Z₀, D), Q (Z=Z₁, D) and Q (Z=Z₂, D). Thenegative charges on the secondary anode or pixel produced when chargesarrive at the primary pixel provide a negative value of secondary signalthat may be utilized to determine an energy correction factor asdiscussed herein. For example, the value (or the magnitude) of the valueof Q for a given curve at the location on the horizontal axis where Z=Dmay be used as the negative value as discussed herein. As seen in FIG.16C, it may be noted that the secondary signal (charge Q, energy E orsecondary induced signal S) depends on the DOI, such that it isdifferent for each different DOI. The negative value S of the signal Qor E is minimum and is equal to zero for events occurring at Z=0. Insuch a case, the energy of the primary signal measured by pixel 1210 ismaximal and is equal to the peak-energy E_(P) in the spectrum of pixel1210 of the radiation detector assembly 1200. The deeper the DOI of agiven event, the higher in magnitude the negative value S of the inducedsignal for the given event will be. It should be noted that, wherereference is made to the secondary induced charge Q, it can alsocorrespond or refer to the secondary energy signal E or S since Q, E,and S are proportional to each other. Accordingly, it may be noted thatreferences to charge Q are also generally similar to references energy Eor S and vice versa.

From graphs 1242 (FIG. 16B) and 1250 (FIG. 16C), it can be seen that foreach DOI there is a corresponding value of the primary signal Q or E(graph 1242) and also a corresponding negative value of the inducedsecondary signal S, Q or E (graph 1250). Accordingly, the values of theprimary signals E (graph 1242) are correlated with the negative valuesof the secondary induced signals S (graph 1250). In various embodiments,the relationship between the value of the primary signal and thenegative value of the secondary signal (or signals) may be used todetermine an energy correction factor and/or to adjust an energy levelof detected events to improve sensitivity and/or resolution.

The correlation may be expressed by the correlation function F writtenin equation (5) above and reproduced below:

E=F(S)  Eq(5)

As also indicated above, the correlation function F(S) that gives thevalue of the primary peak-energy E_(p) measured in the primary pixel1210 as a function of the negative values S of the induced secondarysignal measured on the anode of pixel 1208 is not an analytical functionbut instead is derived empirically in various embodiments.

FIG. 17 provides a graph 1300 containing example empirical functionsF(S) that may be derived from the radiation detector assembly 1200 ofFIG. 16A by measuring and correlating the values of the primary signalsE shown by graph 1242 (FIG. 16B) and the negative values S of thesecondary induced signals shown by graph 1250 (FIG. 16C). For eachsecondary negative value S measured on the anode of the secondary pixel1208 (graph 1250), multiple measurements are done for the primarysignals E measured on the anode of the primary pixel 1210 (graph 1242).The multiple primary signals E measured for a certain negative value Srepresent the spectrum of primary pixel 1210. The maximum value ofsignals E in the spectrum of the primary pixel 1210 measured for acertain negative value S is selected and represents the peak-energy Erinthe spectrum of primary pixel 1210 for a certain negative value ofsecondary induced signal measured by adjacent pixel 1208.

As seen in FIG. 17, the graph 1300 shows three different examples (curve1302, curve 1304 and curve 1306) of the primary peak-energy E_(P) versusthe negative value S of the corresponding secondary induced signal forthree different example aspect ratios between the width W of pixels1206, 1208, 1210 of the radiation detector assembly 1200 and thethickness D of the radiation detector assembly 1200 (e.g., thickness ofsemiconductor 1202 in FIG. 16A). For curve 1302, W/D=0.2. For curve1304, W/D=1. For curve 1306, W/D=1. Curves 1302 and 1304 correspond tononuniform internal electrical fields, while curve 1306 corresponds to ahypothetical situation including a uniform internal electrical field.

As seen in FIG. 17, the maximum measured peak-energy E_(P0)(DOI₀) isderived when S=0. For any other value S<0, the measured EP<E_(P0)(DOI₀).For example, point 1308 on curve 1304 is the primary peak-energyE_(P1308)<E_(P0)(DOI₀) in the spectrum derived for the negative value ofthe induced signal S<0. To correct primary peak-energy E_(P1308) tobring its value to be equal to E_(P0)(DOI₀), the primary peak-energyE_(P1308) is multiplied in various embodiments by a correction factor Kthat is given as follows:

K=E _(P0)(DOI₀)/E _(P1308)(DOI₁₃₀₈); or  Eq(7)

K(S)=E _(P0)(S ₀)/E _(P1308)(S ₁₃₀₈)  Eq(8)

where E_(P0)(S₀) and E_(P1)(S₁₃₀₈) are the peak-energies correspondingto negative values S₀ and S₁₃₀₈. Accordingly, in various embodiments,the energy correction factor may be determined using a plot of thenegative value against the value of the primary signal. The value ofcorrection factor K is also equal to the ratio between the lengths oflines 1312 and 1310 in various embodiments. Accordingly, the energycorrection factor may be defined by the relationship between a magnitudeof the primary value and the value of the negative signal correspondingto the ratio between the peak-energy E_(P0) (S₀) for DOI₀=0 and S₀=0 andthe peak energy E_(P1308)(S₁₃₀₈) for DOI₁₃₀₈ and S₁₃₀₈ (e.g., length ofline 1312 and length of line 1310). Point 1308 on curve 1306 is used,for example, to demonstrate how the value of correction factor K may becalculated. Such calculations for factor K can be done for correctingany point on curves 1302, 1304, or 1306 for correcting their values ofthe primary peak-energy E_(P) to make it equal (or approximately equal)to primary peak-energy E_(P0) (DOI₀).

The peak-energy E_(P) of curve 1302 (with the relatively lower aspectratio of 0.2 compared to the other curves 1304, 1306) depends relativelyweakly on the negative value S since the aspect ratio W/D=0.2 results ina relatively strong small-pixel effect that reduces the dependency onthe DOI and thus on S relative to the curves corresponding to an aspectratio of 1.

In contrast, the peak-energy E_(P) of curve 1306 more strongly dependson the negative value S since the aspect ratio is higher (W/D=1) anddoes not produce a significant small-pixel effect. Accordingly for curve1306, there is a relatively strong dependency on the DOI and thus on S.

The peak-energy E_(P) of curve 1304 moderately depends (e.g., lessstrongly than curve 1306 and more strongly than curve 1302) on thenegative value S since the aspect ratio is 1, and does not produce asignificant small-pixel effect. However, the nonuniform internalelectrical field in the detector may produce an effect somewhat similarto the like small-pixel effect (see, e.g., U.S. Pat. No. 9,954,132entitled “Systems and Methods for Detectors Having Improved InternalElectrical Fields” issued Apr. 24, 2018.) Accordingly, the curve 1304has a relatively moderate dependency on the DOI and thus on S (e.g.,less of a dependency than exhibited by curve 1306).

It may be noted that, alternatively, the correction factor K(S) ofequation (8) above may be derived in various embodiments as explainedbelow, with reference to FIG. 10 discussed above. As discussed above,FIG. 10 schematically illustrates a calibration system 92 that may beused to determine a correlation function of negative values S versus theDOI as shown by equation (2), reproduced as follows:

S=g(DOI)  Eq(2)

The radiation source 97 of calibration system 92 may scan the detector93 by irradiating the side-wall of detector 93 at different DOI's toacquire detector spectra for different values of DOI. For each DOI,there is a specific negative value S of the induced secondary signal,which is common to all the events in the spectrum acquired at a certainDOI for a primary pixel. For each DOI, a detector spectrum may beacquired, in a primary pixel, from which the peak-energy E_(P) isderived. Accordingly, for each DOI, a correlation between thepeak-energy E_(P) and the negative secondary signal S may be derived. Byscanning the detector for various values of DOI, a correlation functionof E_(P) versus S may be derived to empirically measure the correlationfunction F(S) shown in equation (5) (i.e., E=F(S)).

In various embodiments, the correction factor K(S) may be derived by theratio between the energies received from equation (5) for the values S₀and S₁₃₀₈ of graph 1300 in FIG. 17 to yield equation (8), reproducedagain below:

K(S)=E _(P0)(S ₀)/E _(P1308)(S ₁₃₀₈)  Eq(8)

As explained in more detail in connection with the descriptions belowassociated with FIGS. 18 and 19, applying energy correction factors asdiscussed herein to the spectrum produced by the radiation detectorassembly 1200 improves the energy-resolution by narrowing the peak ofthe spectrum, and improves the sensitivity by reducing the number ofevents in the low-energy tail of the spectrum by moving events from thetail into the peak of the spectrum.

FIG. 18 schematically illustrates multiple spectra 1400 of the radiationdetector assembly 1200. Spectrum 1402 of radiation detector assembly1200 shows a histogram of the number of counts N (number of events)versus the energy E of the events. Spectrum 1402 may be understood as asuper-positioning of unobserved multiple sub-spectra 1404, 1406, 1408,1410, with each of the sub-spectra corresponding to a certain andspecific negative value S of the secondary induced signal. It may benoted that only part of the multiple sub-spectra that may make upspectrum 1402 are shown in FIG. 18 for clarity and ease of illustration.In the illustrated example, each of the sub-spectra 1404, 1406, 1408,1410 corresponds to negative specific values S of the induced secondarysignals S₀=0, S₁, S₂ and S respectively. (It may be noted that eachsub-spectra need not be limited to only a single isolated value or rangeof S). For example, each sub-spectra may include a relatively narrowrange of values corresponding to a single nominal value of S.)

As seen in FIG. 18, the values of the peak-energies E_(P) 1412, 1414 and1416 marked as E_(P0), E_(P1) and E_(P2) of spectra 1404, 1406 and 1408,respectively, have magnitudes that trend lower with the negative valueof S (e.g., the more negative is S the lower is the energy E of thepeak-energy E_(P)). As also seen in FIG. 18, the number of counts/eventsN in spectra 1404, 1406 and 1408, corresponding to S₀=0, S₁ and S₂,respectively, tends to go down with the negative value of S (e.g., themore negative is S the lower is number of N in the spectrum). This isdue to the negative value S becoming more negative due to the value ofthe DOI, and the absorption of the events in the region of the DOI goesdown with the value of the DOI. Accordingly, the magnitude of thenegative value of S goes up (becomes more negative) with the value ofthe DOI, but N goes down with the value of the DOI. Accordingly, N goesdown with the negative value of S.

The sub-spectra 1404, 1406, 1408, 1410 have different positions of theirpeak-energies for the different values of S. As such, the sub-spectraare shifted relative to each other. The more negative is the value Scorresponding to a given sub-spectrum, the larger is the peak shifttoward the lower energies for that given sub-spectra. In the illustratedexample, the shift between sub-spectra 1404 and 1406 is relativelysmall, but contributes to the broadening of the peak of spectrum 1402,and accordingly degrades the energy resolution of radiation detectorassembly 1200. As another example, the shift between sub-spectra 1404and 1408 is relatively large and contributes to the increase in thenumber of events in the low-energy tail of spectrum 1402, andaccordingly degrades the sensitivity of radiation detector assembly1200. As such, sub-spectra corresponding to small negative values S tendto degrade the energy resolution of the spectrum 1402 of radiationdetector assembly 1200 while, sub-spectra corresponding to largenegative values S tend to degrade the sensitivity of spectrum 1402 ofradiation detector assembly 1200.

Sub-spectra such as spectra 1404, 1406, 1408, 1410 that are notobservable conventionally (i.e., by obtaining a histogram of the numberof events N versus the energy E of the events), may be revealed andobtained as described above in connection with Figues 16A-C.Accordingly, in various embodiments, sub-spectra may be acquired bybreaking down spectrum 1402 into its component sub-spectra by dividingthe events of spectrum 1402 into multiple groups, with the events ineach group corresponding to a similar negative value S, and each one ofthe groups corresponding to a different negative value S. Each group ofthese groups represents a sub-spectra corresponding to a certain value S(e.g., a range centered on a particular value) that is different fromthe other groups. After sub-spectra have been obtained, the correctionfactors K, derived for example as discussed in connection with FIG. 17,may be applied on the sub-spectra to improve the energy-resolution andsensitivity of radiation detector assembly 1200, for example as shownand discussed in connection with FIG. 19.

FIG. 19 provides a graph 1500 illustrating an example of application ofa correction factor K on the energies of the events in sub-spectra forimproving the energy-resolution and the sensitivity of radiationdetector assembly 1200. It may be noted that the same sub-spectra 1404,1406, 1408, 1410 of FIG. 18 appear in FIG. 19, but in differentenergy-positions of their peak-energy. Accordingly, the same referencenumerals are used in FIGS. 18 and 19 for the sub-spectra. Similarly, thesame reference numeral 1402 is used for the uncorrected spectrum 1402 ofFIG. 18 and the corrected spectrum 1402 in FIG. 19. Similarly,peak-energy E₀ is marked 1412 in both figures.

Generally, events in each sub-spectra have corresponding value S that isdifferent than the value for the other sub-spectra. Accordingly, eventsin each particular sub-spectra may be corrected by a specific correctionfactor K(S) corresponding to the specific value of S that belongs tothat particular sub-spectra. After applying the appropriate correctionfactors K(S) on each sub-spectra 1404, 1406, 1408, 1410, the sub-spectrakeep their shape but all of them are shifted to align their peak-energywith peak-energy E₀. The energy position of peak-energy E₀ is the same,prior and post correction since the correction factor K is equal to 1for this peak-energy.

As seen in FIG. 19, the super position of the sub-spectra generatesspectrum 1402 with peak-energy E₀,, improving energy-resolution andsensitivity compared to the uncorrected spectrum 1402 of FIG. 18. Forexample, corrected spectrum 1402 of FIG. 19 has a peak that is narrowerthan the peak of the un-corrected spectrum 1402 in FIG. 18 andaccordingly has improved energy resolution. Similarly, the correctedspectrum 1402 of FIG. 19 has a lower low-energy tail (or less events inthe tail) and accordingly improved sensitivity since the energycorrection (energy shift) moves events from the low-energy tail into thepeak of the spectrum (or from outside of a window used for countingevents to inside a window used for counting events). The peak ofspectrum 1402 in FIG. 19 includes more events than the peak of spectrum1402 in FIG. 18 since in FIG. 19 all the peaks of the sub-spectra arealigned to the same peak energy 1412 (E₀=Ep₀) This alignment also causesto the peak of spectrum 1402 in FIG. 19 to appear at higher peak-energythan the peak-energy of spectrum 1402 in FIG. 18.

It may be noted that general principles discussed herein may be appliedon a sub-pixel basis. For example, in various embodiments, theprocessing unit 120 is configured to determine energy correction factorsfor sub-pixels of the pixelated anodes, and to apply the energycorrection factors on a sub-pixel basis. Additionaly details regardingthe use of sub-pixels may be found in U.S. Pat. No. 9,927,539 (“Systemsand Methods for Improving Imaging by Sub-Pixel Calibration”), referredto herein as the 539 Patent, the entire subject matter of which ishereby incorporated.

It may be noted that the calibration and/or correction techniquesdiscussed herein to improve energy resolution and/or reduce the lowenergy tail may also be useful for sub-pixels as well. For example,various calibration techniques disclosure herein may be applied based onsub-pixels containing the events of the primary pixels, with locationsin X and Y directions inside the primary pixels derived using thesecondary signals. While the techniques discussed herein generallyrelate to energy calibration and/or correction for different DOI's oralong the Z direction, the techniques disclosed in the 539 patent relateto energy calibration and/or correction along the lateral or X and Ydirections (e.g., to account for in homogeneities with the pixels alongthe lateral directions).

Accordingly, in various embodiments, techniques from the 539 patent maybe used to correct energies in sub-pixels along the lateral directions,and techniques disclosed herein may be used to correct energies insub-pixels along the vertical or Z direction. The combination oftechniques may be used to yield three dimensional energy corrections forsub-pixels. For example, the negative value S and the values of theinner coordinates X,Y of the appropriate sub-pixel may be assigned toeach event. It may be noted that the corrections may be performed in thevertical direction first and subsequently for the lateral directions, orvice versa.

In various embodiments, the corresponding values of S, X, and Y areassigned for each event. Accordingly, each event may be understood asbeing tagged by the values of the parameters (S, X, Y). Correction maythen be applied as follows. All the events may be divided into differetgroups such that each group includes events with different values (S, X,Y), and the events within the same group includes events with similar(S, X, Y) (e.g., events for a certain group are included withinrelatively narrow ranges of S, X, and Y, respectively). Next, a spectrumis produced for each group of events, and the energy-peak of each groupof events is derived. Next, the energy-peak with the maximal energyvalue out of all the energy values of the energy-peaks of the differentspectra of the different grous are derived and defined as the maximumenergy-peak. The calibraton factor K for each group of events may thenbe defined as the ratio between the energy value of the maximumenergy-peak and the energy value of the energy-peak of the group.

FIG. 20 provides a flowchart of a method 2000 (e.g., for determining anenergy correction factor as discussed herein), in accordance withvarious embodiments. The method 2000, for example, may employ or beperformed by structures or aspects of various embodiments (e.g., systemsand/or methods and/or process flows) discussed herein. In variousembodiments, certain steps may be omitted or added, certain steps may becombined, certain steps may be performed concurrently, certain steps maybe split into multiple steps, certain steps may be performed in adifferent order, or certain steps or series of steps may be re-performedin an iterative fashion. In various embodiments, portions, aspects,and/or variations of the method 2000 may be able to be used as one ormore algorithms to direct hardware (e.g., one or more aspects of theprocessing unit 120 and/or processing unit 1290) to perform one or moreoperations described herein.

At 2002, primary signals and secondary signals are acquiredcorresponding to acquisition events (e.g., events corresponding toreception of photons). The primary and secondary signals are generatedresponsive to reception of photons by a semiconductor detector, and arereceived from pixelated anodes (e.g., anodes of a semiconductor deviceof an imaging system such as assembly 100 or assembly 1200). Forexample, a patient that has been administered at least oneradiopharmaceutical may be placed within a field of view of one or moredetectors, and radiation (e.g., photons) emitted from the patient mayimpact the pixelated anodes disposed on reception surfaces of the one ormore detectors resulting in acquisition events (e.g., photon impacts).For a given photon impact in the depicted example embodiment, a primarysignal (responsive to a collected charge) is generated by the impactedpixelated anode (or collecting anode), and one or more secondary signals(responsive to non-collected charge) are generated by pixelated anodesadjacent to the impacted pixelated anode (or non-collecting anodes). Thesecondary signals defining corresponding negative values as discussedherein.

At 2004, an energy correction factor for the semiconductor device isdetermined for acquisition events resulting in the primary and secondarysignals acquired at 2002. In various embodiments, the energy correctionfactor for a given event is determined using at least one secondarysignal for that particular event. For example, as discussed herein, theenergy correction factor in various embodiments is determined using anegative value for at least one secondary signal from one or moreadjacent or non-collecting pixels (e.g., at least one adjacent pixelatedanode). For example, the energy correction factor for each event may beidentified based on the negative value of a secondary signal for thatevent. It may be noted that the energy correction factor in variousembodiments is determined without using any information (e.g., detectedcharge or corresponding signals) from a cathode of the detectorassembly.

In various embodiments, the energy correction factors may be determinedusing calibration information. For example, in the depicted embodiment,at 2006, calibration information is used to determine the energycorrection factor. For example, for each event, an appropriate energycorrection factor may be identified based on the negative value of thesecondary signal for the event. In various embodiments, the energycorrection factors for different negative values may be predetermined aspart of a calibration process. For example, the energy correction factorfor a given event may be identified using a calibration based on a ratiobetween a negative value of at least one secondary signal for the eventand a corresponding amplitude or magnitude of the primary signal.

At 2008, a sub-pixel location is determined using the primary signal andthe at least one secondary signal. For each event, a correspondingsub-pixel location may be determined. It may be noted that the sameinformation (primary and secondary signals) used to determine energycorrection factors for events may also be used to determine sub-pixellocations for those events.

At 2010, an energy level for an event is adjusted using the appropriateenergy correction factor. In various embodiments, as discussed herein,the energy correction factor is selected using the negative value of asecondary signal for the event. The energy levels are adjusted invarious embodiments to make the energy levels for a group of events moreconsistent and/or closer to a target (e.g., a maximal peak energy).

At 2012, an image is reconstructed using the adjusted energy levels ofthe events (e.g., using counts of events within a predetermined windowof energy levels after adjusting the energy levels).

As discussed herein, a radiation detector system may be calibrated toprovide predetermined energy correction factors based on negative valuesof secondary signal for events. FIG. 21 provides a flowchart of a method2100 (e.g., for providing and calibrating a radiation detectorassembly), in accordance with various embodiments. The method 2100, forexample, may employ or be performed by structures or aspects of variousembodiments (e.g., systems and/or methods and/or process flows)discussed herein. In various embodiments, certain steps may be omittedor added, certain steps may be combined, certain steps may be performedconcurrently, certain steps may be split into multiple steps, certainsteps may be performed in a different order, or certain steps or seriesof steps may be re-performed in an iterative fashion. In variousembodiments, portions, aspects, and/or variations of the method 2100 maybe able to be used as one or more algorithms to direct hardware (e.g.,one or more aspects of the processing unit 120 and/or processing unit1290) to perform one or more operations described herein.

At 2102, a semiconductor detector (e.g., semiconductor 1202 of radiationdetector assembly 1200) is provided. The semiconductor detector of theillustrated example has a surface with plural pixelated anodes disposedon the surface. Each pixelated anode is configured to generate a primarysignal responsive to reception of a photon by the pixelated anode and togenerate at least one secondary signal responsive to an induced chargecaused by reception of a photon by at least one adjacent anode. At 2104,the pixelated anodes are operably coupled to at least one processor(e.g., processing unit 1290).

At 2106, a calibrated radiation supply (e.g., having a known photonenergy) is provided. In some embodiments, the calibrated radiationsupply is provided at different depths along a sidewall of thesemiconductor detector. Responsive to reception of the calibratedradiation supply, the pixelated anodes generate primary and secondarysignals. For example, the calibrated radiation supply may be passedthrough a pin-hole collimator to the sidewall of the semiconductordetector. The location of a given pin-hole (e.g., in a Z direction)through which radiation is passed may be used to determine the DOI atwhich the corresponding radiation is passed from the collimator andreceived by the semiconductor detector, and events may be grouped byDOI. Alternatively, events may be generated at unknown depths andgrouped based on a negative value generated by a secondary signal forthe event. At 2108, the primary and secondary signals are acquired fromthe pixelated anodes by the at least one processor.

At 2110, corresponding negative values of total induced charges (e.g.,corresponding to the various different depths at which the radiation hasbeen absorbed) are determined. At 2112, calibration information (e.g., alook-up table or other correlating relationship between negative inducednon-collected charge values and energy correction factors) isdetermined. For example, for events having a given negative value X(e.g., negative values within a predetermined range of X), relationshipsbetween the magnitude of the primary signal and the magnitude of thenegative value may be used to define a correction factor that may beused to adjust the energy levels of events having the given negativevalue X.

It should be noted that the various embodiments may be implemented inhardware, software or a combination thereof. The various embodimentsand/or components, for example, the modules, or components andcontrollers therein, also may be implemented as part of one or morecomputers or processors. The computer or processor may include acomputing device, an input device, a display unit and an interface, forexample, for accessing the Internet. The computer or processor mayinclude a microprocessor. The microprocessor may be connected to acommunication bus. The computer or processor may also include a memory.The memory may include Random Access Memory (RAM) and Read Only Memory(ROM). The computer or processor further may include a storage device,which may be a hard disk drive or a removable storage drive such as asolid-state drive, optical disk drive, and the like. The storage devicemay also be other similar means for loading computer programs or otherinstructions into the computer or processor.

As used herein, the term “computer” or “module” may include anyprocessor-based or microprocessor-based system including systems usingmicrocontrollers, reduced instruction set computers (RISC), ASICs, logiccircuits, and any other circuit or processor capable of executing thefunctions described herein. The above examples are exemplary only, andare thus not intended to limit in any way the definition and/or meaningof the term “computer”.

The computer or processor executes a set of instructions that are storedin one or more storage elements, in order to process input data. Thestorage elements may also store data or other information as desired orneeded. The storage element may be in the form of an information sourceor a physical memory element within a processing machine.

The set of instructions may include various commands that instruct thecomputer or processor as a processing machine to perform specificoperations such as the methods and processes of the various embodiments.The set of instructions may be in the form of a software program. Thesoftware may be in various forms such as system software or applicationsoftware and which may be embodied as a tangible and non-transitorycomputer readable medium. Further, the software may be in the form of acollection of separate programs or modules, a program module within alarger program or a portion of a program module. The software also mayinclude modular programming in the form of object-oriented programming.The processing of input data by the processing machine may be inresponse to operator commands, or in response to results of previousprocessing, or in response to a request made by another processingmachine.

As used herein, a structure, limitation, or element that is “configuredto” perform a task or operation is particularly structurally formed,constructed, or adapted in a manner corresponding to the task oroperation. For purposes of clarity and the avoidance of doubt, an objectthat is merely capable of being modified to perform the task oroperation is not “configured to” perform the task or operation as usedherein. Instead, the use of “configured to” as used herein denotesstructural adaptations or characteristics, and denotes structuralrequirements of any structure, limitation, or element that is describedas being “configured to” perform the task or operation. For example, aprocessing unit, processor, or computer that is “configured to” performa task or operation may be understood as being particularly structuredto perform the task or operation (e.g., having one or more programs orinstructions stored thereon or used in conjunction therewith tailored orintended to perform the task or operation, and/or having an arrangementof processing circuitry tailored or intended to perform the task oroperation). For the purposes of clarity and the avoidance of doubt, ageneral purpose computer (which may become “configured to” perform thetask or operation if appropriately programmed) is not “configured to”perform a task or operation unless or until specifically programmed orstructurally modified to perform the task or operation.

As used herein, the terms “software” and “firmware” are interchangeable,and include any computer program stored in memory for execution by acomputer, including RAM memory, ROM memory, EPROM memory, EEPROM memory,and non-volatile RAM (NVRAM) memory. The above memory types areexemplary only, and are thus not limiting as to the types of memoryusable for storage of a computer program.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the variousembodiments without departing from their scope. While the dimensions andtypes of materials described herein are intended to define theparameters of the various embodiments, they are by no means limiting andare merely exemplary. Many other embodiments will be apparent to thoseof skill in the art upon reviewing the above description. The scope ofthe various embodiments should, therefore, be determined with referenceto the appended claims, along with the full scope of equivalents towhich such claims are entitled. In the appended claims, the terms“including” and “in which” are used as the plain-English equivalents ofthe respective terms “comprising” and “wherein.” Moreover, in thefollowing claims, the terms “first,” “second,” and “third,” etc. areused merely as labels, and are not intended to impose numericalrequirements on their objects. Further, the limitations of the followingclaims are not written in means-plus-function format and are notintended to be interpreted based on 35 U.S.C. §112(f) unless and untilsuch claim limitations expressly use the phrase “means for” followed bya statement of function void of further structure.

This written description uses examples to disclose the variousembodiments, including the best mode, and also to enable any personskilled in the art to practice the various embodiments, including makingand using any devices or systems and performing any incorporatedmethods. The patentable scope of the various embodiments is defined bythe claims, and may include other examples that occur to those skilledin the art. Such other examples are intended to be within the scope ofthe claims if the examples have structural elements that do not differfrom the literal language of the claims, or the examples includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. A radiation detector assembly comprising: asemiconductor detector having a surface; plural pixelated anodesdisposed on the surface, each pixelated anode configured to generate aprimary signal responsive to reception of a photon by the pixelatedanode and to generate at least one secondary signal responsive to aninduced charge caused by reception of a photon by at least onesurrounding anode; and at least one processor operably coupled to thepixelated anodes, the at least one processor configured to: acquire aprimary signal from one of the anodes responsive to reception of aphoton by the one of the anodes; acquire at least one secondary signalfrom at least one neighboring pixel of the one of the anodes responsiveto an induced charge caused by the reception of the photon by the one ofthe anodes, the at least one secondary signal defining a negative value;and determine an energy correction factor for the reception of thephoton by the one of the anodes using the negative value of the at leastone secondary signal.
 2. The radiation detector assembly of claim 1,wherein the at least one processor is configured to adjust an energylevel of detected events using the energy correction factor.
 3. Theradiation detector assembly of claim 1, wherein the at least oneprocessor is configured to use a relationship of a value of the primarysignal with the negative value of the at least one secondary signal todetermine the energy correction factor.
 4. The radiation detectorassembly of claim 3, wherein the at least one processor is configured todetermine the energy correction factor using a plot of the negativevalue against the value of the primary signal.
 5. The radiation detectorassembly of claim 1, wherein the energy correction factor is defined bya relationship between a magnitude of the primary signal and a magnitudeof the negative value.
 6. The radiation detector assembly of claim 1,wherein the at least one processor is configured to apply the energycorrection factor to move at least one detected event from outside of anenergy window used for counting to inside the energy window used forcounting.
 7. The radiation detector assembly of claim 1, wherein the atleast one processor is configured to organize detected events intogroups based on magnitudes of corresponding negative values, and toapply corresponding energy correction factors to the groups.
 8. Theradiation detector assembly of claim 1, wherein the at least oneprocessor is configured to define sub-spectra based on magnitudes ofcorresponding negative values for detected events, and align peaks ofthe sub-spectra using corresponding energy correction factors.
 9. Theradiation detector assembly of claim 1, wherein the at least oneprocessor is configured to determine energy correction factors forsub-pixels of the pixelated anodes, and to apply the energy correctionfactors on a sub-pixel basis.
 10. A method of imaging using asemiconductor detector having a surface with plural pixelated anodesdisposed thereon, wherein each pixelated anode is configured to generatea primary signal responsive to reception of a photon by the pixelatedanode and to generate at least one secondary signal responsive to aninduced charge caused by reception of a photon by at least onesurrounding anode, the method comprising: acquiring a primary signalfrom one of the anodes responsive to reception of a photon by the one ofthe anodes; acquiring at least one secondary signal from at least oneneighboring pixel of the one of the anodes responsive to an inducedcharge caused by the reception of the photon by the one of the anodes,the at least one secondary signal defining a negative value; anddetermining an energy correction factor for the reception of the photonby the one of the anodes using the negative value of the at least onesecondary signal.
 11. The method of claim 10, further comprisingadjusting an energy level for an event corresponding to the reception ofthe photon by the one of the anodes using the energy correction factor.12. The method of claim 10, further comprising reconstructing an imageusing the adjusted energy level.
 13. The method of claim 10, furthercomprising using calibration information to determine the energycorrection factor.
 14. The method of claim 13, further comprisingdetermining the energy correction factor using a calibration based on arelationship between a magnitude of the primary signal and the negativevalue of the secondary signal.
 15. The method of claim 10, furthercomprising determining the energy correction factor without using anyinformation from a cathode of the detector assembly.
 16. The method ofclaim 10, further comprising determining a sub-pixel location using theprimary signal and the at least one secondary signal.
 17. The method ofclaim 10, further comprising using a relationship of a value of theprimary signal with the negative value of the at least one secondarysignal to determine the energy correction factor.
 18. The method ofclaim 10, further comprising determining the energy correction factorusing a plot of the negative value against the value of the primarysignal.
 19. A method of providing a radiation detector assemblycomprising: providing a semiconductor detector having a surface withplural pixelated anodes disposed thereon, each pixelated anodeconfigured to generate a primary signal responsive to reception of aphoton by the pixelated anode and to generate at least one secondarysignal responsive to an induced charge caused by reception of a photonby at least one adjacent anode; operably coupling the pixelated anodesto at least one processor; providing a calibrated radiation supply tothe semiconductor detector, wherein the pixelated anodes generateprimary signals and secondary signals responsive to the calibratedradiation supply; acquiring, with the at least one processor, theprimary signals and the secondary signals from the pixelated anodes;determining corresponding negative values of total induced charges forthe secondary signals corresponding to different depths of absorption;determining calibration information based on the negative values of thetotal induced corresponding to the different depths.
 20. The method ofclaim 19, wherein the calibrated radiation supply is provided along asidewall of the semiconductor detector at different depths.
 21. Themethod of claim 19, wherein the calibration information includes energycorrection factors based on corresponding relationships between amagnitude of the primary signal and the negative value of the secondarysignal.